The chart shows the 16 major greyscales (of the 255 available).
If your monitor is correctly adjusted you should be able to see all the shades on this chart.

If you can only see differences in the mid and lower shades your brightness is too high,
if you can only see a difference between the upper and mid shades the brightness is probably
too low.

If you can only get the middle shades to differentiate reduce the contrast until they're
all visible. If you still can't get all the boxes to appear check that you have a 24bit or
32bit display mode (also known as a true-color display mode) enabled.